Molybdenum is principally found in the earth's crust in the form of molybdenite (MoS.sub.2) distributed as very fine veinlets in quartz which is present in an ore body comprised predominantly of altered and highly silicified granite. The concentration of the molybdenite in such ore bodies is relatively low, that is, in amounts between about 0.1% and about 0.6%, by weight. The molybdenite is present in the form of relatively soft, hexagonal, black flaky crystals which are extracted from the ore body and concentrated by one of a variety of known beneficiation processes so as to increase the molybdenum disulfide content to a level of usually about greater than 80%, by weight of the concentrate. The resulting concentrate is subjected to an oxidation step, which usually is performed by a roasting operation in the presence of air, whereby the molybdenum disulfide is converted to molybdenum oxide which is of a commercial or technical grade containing various impurities including metallic contaminants present in the original ore body.
It is desirable or necessary to provide in some instances a molybdenum product for molybdate compounds which are relatively free of such metallic contaminants providing a high purity material for use in the preparation of molybdenum compounds, catalysts, chemical reagents or the like, wherein the presence of potassium is detrimental. The production of high purity ammonium molybdate and molybdenum oxide has heretofore been achieved by various chemical and physical refining techniques, of which the sublimation of technical grade oxide at elevated temperatures, e.g., about 1000.degree. C. or higher, and recovering the sublimed molybdenum trioxide of a purity usually of about 99% or greater, is perhaps the most common method. Unfortunately, the sublimation process is inefficient, producing a purified molybdenum trioxide in relatively low yields, necessitating a recycling and further processing of the residue to recover the residual molybdenum values therein, which still further detracts from the economics of the purification process.
Particular attention has been given in recent years to the potassium content of molybdenum trioxide, and generally molybdenum trioxide having less than about 100 parts per million (ppm), e.g. between about 40 ppm and about 80 ppm potassium. Purified oxide of this quality is specified for manufacture of molybdenum powder which goes into the production of wire, rod, sheet, sinter parts and other mill products, special alloys and chemically pure molybdenum salts such as might be used in catalysts.
Typical processes for treating molybdenum disulfide or molybdenum trioxide to produce chemical grade molybdenum trioxide are disclosed in U.S. Pat. Nos. 1,888,978; 1,923,652; 3,139,326; 3,351,423; 3,393,971; 3,848,049; 3,932,580; 3,957,946.